1. Field of the Invention
The invention relates to “an inter-cylinder air-fuel ratio imbalance determining apparatus for an internal combustion engine” that is applied to a multi-cylinder internal combustion engine and determines (monitors/detects) that the imbalance in the air-fuel ratio of an air-fuel mixture supplied to each cylinder (an inter-cylinder air-fuel ratio imbalance, an inter-cylinder variation in air-fuel ratio, non-uniformity in air-fuel ratio among cylinders) becomes excessively large.
2. Description of Related Art
When a fuel injection characteristic of a fuel injection valve for supplying fuel mainly to a specific cylinder (a fuel injection valve for a specific cylinder) is different from that of a fuel injection valve for supplying fuel mainly to another cylinder (a fuel injection valve for another valve), an inter-cylinder air-fuel ratio imbalance state occurs. The fuel injection characteristic is a characteristic indicative of the ratio of the amount of actually injected fuel relative to an instructed fuel injection amount. When the inter-cylinder air-fuel ratio imbalance state has occurred due to the difference in fuel injection characteristic mentioned above, a difference in the air-fuel ratio of exhaust gas discharged from a plurality of cylinders is increased. According to a discharge order (consequently, according to an ignition order), the exhaust gas discharged from a plurality of cylinders of an engine sequentially reaches “an air-fuel ratio sensor disposed in an exhaust collection portion where exhaust gas from the plurality of cylinders is collected (an upstream air-fuel ratio sensor)”. As a result, when the inter-cylinder air-fuel ratio imbalance state has occurred, as shown in FIG. 2, an output of an air-fuel ratio obtained based on the air-fuel ratio sensor (a detected air-fuel ratio, an upstream air-fuel ratio) significantly fluctuates.
To cope with this, one of known inter-cylinder air-fuel ratio imbalance determining apparatuses acquires a value corresponding to a change amount per unit time (a time-differential-value corresponding value, a gradient) of “an output value of an air-fuel ratio sensor or a detected air-fuel ratio” as an imbalance determination parameter. In addition, the determining apparatus compares the acquired imbalance determination parameter with an imbalance determination threshold to determine whether or not the inter-cylinder air-fuel ratio imbalance state has occurred based on the comparison result (e.g., see Japanese Patent Application Publication No. 2011-047332 (JP-2011-047332 A)).
Hereinafter, description will be made of the case where the inter-cylinder air-fuel ratio imbalance has occurred due to the characteristic of a fuel injection valve for a specific cylinder in which the fuel injection valve for the specific cylinder injects more fuel than a fuel injection valve for another cylinder. Hereinbelow, such inter-cylinder air-fuel ratio imbalance is also simply referred to as “rich imbalance”.
FIG. 2 shows a waveform of a detected air-fuel ratio in the case where the rich imbalance has occurred. As can be seen from FIG. 2, when the exhaust gas of a specific cylinder (a cylinder causing the rich imbalance) reaches the air-fuel ratio sensor, the detected air-fuel ratio decreases relatively sharply (see Times t0 to t1). In this case, the number of specific cylinders is one and the number of the other cylinders is two or more (for example, when attention is paid to an in-line four-cylinder engine or one of banks of a V8 engine, the number of the other cylinders is three). Normally, when the rich imbalance has occurred, by the feedback control of the air-fuel ratio, the air-fuel ratios of the other cylinders are controlled to be slightly leaner than the stoichiometric air-fuel ratio, and the average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is maintained at the level of a target air-fuel ratio (e.g., the stoichiometric air-fuel ratio). As a result, when the exhaust gas of the other cylinders sequentially reaches the air-fuel ratio sensor after Time t1, the detected air-fuel ratio increases relatively gradually (see Times t1 to t2).
In order to perform the determination of the rich imbalance with improved accuracy, it is considered that “a negative gradient corresponding value” is acquired based on the magnitude of a time-differential-value corresponding value having a negative value (the magnitude of a negative gradient) among the time-differential-value corresponding values of the detected air-fuel ratio, and it is determined whether or not the inter-cylinder air-fuel ratio imbalance has occurred by determining whether or not the magnitude of the negative gradient corresponding value is larger than an imbalance determination threshold.
On the other hand, a V8 engine shown as an example in FIG. 1 includes first, third, fifth, and seventh cylinders in a left bank LB, and second, fourth, sixth, and eighth cylinders in a right bank RB. Branch portions of an exhaust manifold of the cylinders belonging to the left bank lead to a left-bank exhaust collection portion HK (L). Branch portions of an exhaust manifold of the cylinders belonging to the right bank lead to a right-bank exhaust collection portion HK (R).
A left-bank catalyst 43 is disposed in a left-bank exhaust passage downstream of the left-bank exhaust collection portion HK (L). A left-bank upstream air-fuel ratio sensor 66L is disposed in the left-bank exhaust passage at a position between the left-bank exhaust collection portion HK (L) and the left-bank catalyst 43.
A right-bank catalyst 53 is disposed in a right-bank exhaust passage downstream of the right-bank exhaust collection portion 1-1K (R). A right-bank upstream air-fuel ratio sensor 66R is disposed in the right-bank exhaust passage at a position between the right-bank exhaust collection portion HK (R) and the right-bank catalyst 53.
The ignition order (combustion order, discharge order) of an engine 10 is, e.g., #1, #8, #7, #3, #6, #5, #4, and #2, as shown in FIG. 3. Herein, “#N” denotes an N-th cylinder, and N is an integer of 1 to 8. The interval of the ignition (combustion of an air-fuel mixture) corresponds to a period required for rotation by crank angle of 90°.
In the engine 10 described above, when attention is paid to the left bank, the crank angle from the occurrence of the combustion in the first cylinder to the occurrence of the next combustion in the seventh cylinder is 180°, the crank angle from the occurrence of the combustion in the seventh cylinder to the occurrence of the next combustion in the third cylinder is 90°, the crank angle from the occurrence of the combustion in the third cylinder to the occurrence of the next combustion in the fifth cylinder is 180°, and the crank angle from the occurrence of the combustion in the fifth cylinder to the occurrence of the next combustion in the first cylinder is 270°.
Similarly, when attention is paid to the right bank, the crank angle from the occurrence of the combustion in the eighth cylinder to the occurrence of the next combustion in the sixth cylinder is 270°, the crank angle from the occurrence of the combustion in the sixth cylinder to the occurrence of the next combustion in the fourth cylinder is 180°, the crank angle from the occurrence of the combustion in the fourth cylinder to the occurrence of the next combustion in the second cylinder is 90°, and the crank angle from the occurrence of the combustion in the second cylinder to the occurrence of the next combustion in the eighth cylinder is 180°.
Thus, the intervals of the combustion in each bank are not identical, and therefore the intervals of the arrival of the exhaust gas at the exhaust collection portion and the upstream air-fuel ratio sensor (66L, 66R) in each bank are not identical.
On the other hand, a change in the output value of the air-fuel ratio sensor lags behind “a change in the air-fuel ratio of the exhaust gas reaching the air-fuel ratio”. Accordingly, when time from the arrival of “the exhaust gas from one cylinder” at the vicinity of the air-fuel ratio sensor to the arrival of “the exhaust gas from another cylinder” at the vicinity of the air-fuel ratio sensor is short, “the exhaust gas of another cylinder” reaches the air-fuel ratio sensor before the output value of the air-fuel ratio sensor is reduced to the value corresponding to the air-fuel ratio of “the exhaust gas of one cylinder”, and the output value of the air-fuel ratio sensor starts to increase. As a result, for example, as shown in FIG. 4, even when the fuel injection characteristic of the fuel injection valve is changed in the same manner, the magnitude of the negative gradient corresponding value is changed depending on “which cylinder the fuel injection valve belongs to”.
More specifically, since the crank angle from the arrival of the exhaust gas of the fifth cylinder at the air-fuel ratio sensor to the arrival of the exhaust gas of the cylinder subsequent to the fifth cylinder in the discharge order (i.e., the first cylinder) at the air-fuel ratio sensor is 270°, the exhaust gas of the fifth cylinder stays around the air-fuel ratio sensor for a relatively long time period. Therefore, the negative gradient corresponding value becomes relatively large when the injection characteristic of the fuel injection valve of the fifth cylinder is changed.
In contrast to this, since the crank angle from the arrival of the exhaust gas of the first cylinder at the air-fuel ratio sensor to the arrival of the exhaust gas of the cylinder subsequent to the first cylinder in the discharge order (i.e., the seventh cylinder) at the air-fuel ratio sensor is 180° and, similarly, the crank angle from the arrival of the exhaust gas of the third cylinder at the air-fuel ratio sensor to the arrival of the exhaust gas of the cylinder subsequent to the third cylinder in the discharge order (i.e., the fifth cylinder) at the air-fuel ratio sensor is 180°, the negative gradient corresponding value has a medium magnitude when the injection characteristic of the fuel injection valve of the first or third cylinder is changed. In addition, since the crank angle from the arrival of the exhaust gas of the seventh cylinder at the air-fuel ratio sensor to the arrival of the exhaust gas of the cylinder subsequent to the seventh cylinder in the discharge order (i.e., the third cylinder) at the air-fuel ratio sensor is 90°, the exhaust gas of the seventh cylinder can stay around the air-fuel ratio sensor only for a short time period. Therefore, the negative gradient corresponding value becomes relatively small when the injection characteristic of the fuel injection valve of the seventh cylinder is changed.
As can be seen from this, for example, when the fuel injection characteristic of the fuel injection valve of the fifth cylinder is not changed to such a degree that it should be determined that “the inter-cylinder air-fuel ratio imbalance state has occurred”, but the fuel injection characteristic thereof is a characteristic in which a slightly excessive amount of fuel is injected within the design tolerance of the fuel injection valve, the magnitude of the negative gradient corresponding value becomes large to some extent. The magnitude of the negative gradient corresponding value in this case takes a value extremely close to the negative gradient corresponding value obtained when the fuel injection characteristic of the fuel injection valve of the seventh cylinder is changed to such a degree that it should be determined that “the inter-cylinder air-fuel ratio imbalance state has occurred”.
More specifically, as shown in FIG. 5, the magnitudes of the negative gradient corresponding values at points P1 and P2 at which it should be determined that the inter-cylinder air-fuel ratio imbalance state has occurred and the magnitude of the negative gradient corresponding value at a point P3 at which it should not be determined that the inter-cylinder air-fuel ratio imbalance state has occurred are almost the same. Therefore, when the magnitude of the negative gradient corresponding value indicated by a broken line is set as the imbalance determination threshold, it is determined (erroneously determined) that the state of the point P1 or P2 is the state in which the inter-cylinder air-fuel ratio imbalance has not occurred. In contrast to this, when the magnitude of the negative gradient corresponding value indicated by a one-dot chain line is set as the imbalance determination threshold, it is determined (erroneously determined) that the state of the point P3 is the state in which the inter-cylinder air-fuel ratio imbalance has occurred.